Vertical cavity surface emitting laser (VCSEL) has many advantages over edge emitting lasers, such as a simple manufacture process, a single mode as well as multi-mode of operation, high coupling efficiency and lower cost, and etc. So it is widely used in a variety of optical fiber communication system. The VCSELs, indeed a semiconductor laser in general, has the nature that the optical output taken as a function of input current represents a steep slope during lasing action. Hence, during its operations in optoelectronic transmitters, minor variations, e.g. ambient temperature changes and aging of the device, can result in significant fluctuations in its output power. In order to ensure a near constant output power from the VCSEL, a feedback control system is desired, that first requires a monitoring system to detect its optical output. Usually photo detectors are used in this kind of application, with the form of either being integrated into the VCSEL structure or standing alone separately from it.
Integrating monitoring photo detector with VCSEL can only be realized during VCSEL fabrication. More fabrication steps have to be introduced during VCSEL formation and VCSEL normal manufacture process has to be interrupted. That increases the process complicity and the risk for VCSEL yield. It is obvious that these drawbacks can be easily overcome by a stand-alone photo detector; the main consideration can be simplified on how to integrate the detector with VCSEL by packaging.
Packaging the monitoring photo detector with VCSEL together is widely investigated in recent years. Due to that light emitting of a typical VCSEL only comes from one surface of its structure, any monitoring should use the same output for optical communication. Approaches focus on detecting a partial reflection of the output power, which requires a specific assemble position among VCSEL, monitoring photo detector and reflecting system.
To ensure reliable performance of the optical transmitter, it is therefore desired to provide a low cost and non-interfering VCSEL power monitoring system, which can integrate VCSEL with reduced complexity and increased manufacturability.